1. Field of the Invention
This invention relates in general to an apparatus for manipulating the transmission of power. In particular, this invention relates to an apparatus for optimized transmission of mechanical or electrical power produced by a power source to a load, with the transmitted power fulfilling the specific mechanical or electrical power requirement within the full operating speed range of the load.
2. Technical Background
Power is a measure to rate the flow of energy, or work done, per unit time. Electrical power has been conveniently available in households, in factories, and, for example, in the overhead power line system of electrified railway for various applications. On the other hand, mechanical power that is provided as raw motive power by prime movers such as internal combustion engines consuming fossil fuel is also convenient. In practical applications, these are the two forms of readily-available power source in modern society.
The term "power" is used herein as a general term to designate either the mechanical or the electrical form of power. For the discussion of the invention, "power transmission" thus broadly refers to the transmission of mechanical or electrical power to either mechanical or electrical power. Relevant forms of power concerning the manipulation of power transmission as performed by the apparatus of the invention thus include mechanical and electrical power.
When both mechanical and electrical forms of power are considered, there can be four possible modes of power transmission. In conventional terminology, the transmission of mechanical power to mechanical is generally referred as mechanical power transmission, as is in vehicular drive train applications. The transmission of electrical power to mechanical, when utilizing an electric motor, is referred to as electric motoring. In contrast, the transmission of mechanical power to electrical, when employing an electric generator, is electrical power generation. Electrical-to-electrical power transmission generally involves the regulation of voltage and/or frequency of the electrical power. In the extreme case of zero frequency of the AC electrical power, it becomes the DC electrical power.
The need for power transmission is based on one simple reason. Namely, the sources providing the power, either mechanical or electrical, are frequently operating to generate the power at conditions not directly desirable at the load that is consuming the power. Generally, the characteristic factors of power source and load concerned include, in the mechanical power, torque and speed and, in the electrical power, frequency and voltage. For both forms of power, efficiency is a factor of ever increasing importance. For example, considering the vast number of internal combustion engine-driven vehicles used world-wide, small improvements in vehicle power plant and transmission efficiencies can be translated into the huge conservation of petroleum consumption. In critical applications such as electric vehicle, the bottleneck of storage battery technology turns the efficiency of electric drive system into one of the most important design factors.
An internal combustion engine needs a transmission box to provide the torque-speed regulation in order to meet propulsion demands at the vehicle driving wheels. Conventional vehicle internal combustion engine does not provide stall torque, while every vehicle has to be accelerated from standstill. This means that an internal combustion engine which operates within a limited speed range--not including stall speed--must drive the load in a full operating speed range--including the stall speed. The transmission box in an automobile is used to perform this transmission of mechanical power along with the necessary torque/speed regulations. But traditional automobile transmissions built around multiple-speed-geared torque converters suffer drawbacks. They require the use of precision fluid logic valve mechanism to switch the torque converter among the three or more sets of gear train of different gear ratios. The multiple sets of gear trains installed in a typical transmission box, in which only one is functional at any given time, add to the overall weight of the system, and the torque converter operates with poor efficiency at low speeds.
An electric machine operating in the motoring mode--commonly known as the electric motor--does provide stall torque, but with poor efficiency. In large-power electric motor drives, poor starting-speed efficiency imposes heat dissipation problem that the drives must reduce their power rating at low operating speeds in order to prevent permanent damages caused by overheating. Though power electronics devices such as PWM (pulse-width modulation) systems do expand operating speed range and improve motor drive efficiencies, they are generally sophisticated and costly to build.
An electric machine operating in the generating mode as an electric generator is also constrained by input mechanical speed ranges. For example, a wind turbine driving an electric generator has a limitation of minimum wind speed. Below that minimum, the generator system is difficult, if not impossible, to generate an AC power that can be acceptable for house or industrial application.
Thus, when considered as machines for the transmission of the mechanical and/or electrical power in the generalized sense, conventional drive systems, either vehicle transmissions, electric motor drives or generators, all suffer from the low-efficiency performance characteristics at low operating speeds. Yet low-speed operation is a situation inevitable for practically all such power drives. In some situations such as vehicle transmissions operating in traffic congestion conditions, this poor-efficiency performance at low speeds deteriorates air pollution problem at a large scale considering the number of vehicles caught in the traffic. Most of these conventional power transmission machines, though optimized for a fraction, frequently at the high-speed end, of their respective full operating speed range, can not cover the full speed range with optimized performance. Electric motor drives employing digital-controlled power electronics systems can indeed improve overall performance than simple motors in their designed operating speed ranges. However, power electronics motor control systems are sophisticated and expensive to build.
For the foregoing reasons, there is a need for a power transmission apparatus that can transmit power within its full operating speed range with optimized performance characteristics.